1. Field
The present disclosure generally relates to engine valve control systems, and particularly to valve control systems using variable valve timing and variable displacement.
2. Related Art
The optimum times for opening and closing the inlet and exhaust valves in an engine vary, inter alia, with engine speed. In any engine having fixed angles for opening and closing of valves during all engine operating conditions, valve timing is an important design consideration. In many cases, valve timing detracts from engine efficiencies in all but a limited range of operating conditions. For this reason, it has been previously proposed to dynamically vary valve timing during engine operation in order to accommodate different operating conditions.
Hydraulic camshafts are used to regulate valves in an engine combustion chamber. Valve regulation includes both valve timing and valve displacement inside the engine combustion chamber. Valve timing controls both the opening time and the closing time for valves. Valve displacement comprises the distance (lift) that a valve opens and the duration for which the valve is open.
The conventional camshaft-actuated valve gear train is a compromise solution as far as engine efficiency and performance is concerned. For example, at relatively low speeds and loads, the engine valves typically open more than is needed, while at relatively higher engine speeds, the valves typically do not open enough to allow the flow quantity of air-fuel mixture necessary to achieve optimum engine performance. At relatively low speeds, if the amount of valve opening could be reduced, such that the poppet valve could serve as a flow “throttle”, engine pumping losses could be reduced. A poppet valve is an intake or exhaust valve, operated by springs and cams that plugs and unplugs an opening by axial motion.
In some engines, variation of valve timing has been proposed as a means for regulating engine output power. For example, if the inlet valve is allowed to remain open for part of a compression stroke, the volumetric efficiency of an engine can be reduced. Such an engine requires an increased control range over the phase of the hydraulic camshaft. Furthermore, the control needs to be continuous over the full adjustment range.
It has been observed that improvements to engine efficiency can be achieved by varying the timing of the opening and closing of the valves as a function of engine speed, and also as a function of engine load. One known mechanism used to vary the timing of the opening and closing of the engine valves is a variable cam phase change device. The variable cam phase change device is used to vary the angular position of the camshaft, relative to the angular position of the crankshaft.
Various proposals have been suggested for mechanisms used to adjust the camshaft phase angle relative to the crankshaft. However, the suggested mechanisms typically are very complex because of the need to withstand considerable torque fluctuations experienced by a camshaft during normal operation. The camshaft phase angle adjustment mechanism must also supply force sufficient to rotate the camshaft against the resistance provided by the compressed valve springs.
Electro-mechanical valve-actuated systems have been proposed that vary either valve timing or valve displacement. However, it is desirable to simultaneously control both valve timing and valve displacement in a hydraulic valve-actuated system. The present teachings disclose a hydraulic system that varies both valve timing and valve displacement in an engine.